In the prior art, a vacuum microwave thawing machine has been one wherein a reduced-pressure process and a pressure restoration process are repeated, and microwaves are radiated in the reduced-pressure process and thereby foodstuffs to be thawed are heated and thawed. Further, this kind of a conventional vacuum microwave thawing machine is equipped with a chamber for storing foodstuffs to be thawed, a vacuum pump for decompressing the inside of the chamber, recompressing means for the inside of the chamber decompressed by the vacuum pump, and a microwave oscillator for radiating microwaves into the chamber in the reduced-pressure process.
As mentioned above, in the prior art, microwaves are radiated for heating foodstuffs to be thawed only in the reduced-pressure process, as a result, irradiation time of microwaves in one thawing cycle consisting of a reduced-pressure process and a reduced-pressure process had to become short of itself. Therefore, so as to compensate this disadvantage, it has been necessary to start the irradiation of microwaves from a high-pressure range where reduced-pressure is small, or to employ a microwave oscillator that can radiate microwaves of large output, or to radiate microwaves continuously to a low-pressure range where reduced-pressure is large.
While, in the thawing method in a conventional vacuum microwave thawing machine, in the case where irradiation of microwaves is started from a high-pressure range where reduced-pressure is small, pressure restoration time to the pressure restoration upper limit becomes useless, and if reduced-pressure speed changes, irradiation time of microwaves cannot be secured in stable manners, which has been a problem in the prior art.
Further, in the case where a microwave oscillator having large output is employed and thereby the output of microwaves is increased, an edge effect as a phenomenon where microwaves concentrate on the corners of foodstuffs to be thawed is likely to occur, which has been another problem with the prior art.
Further, in the case where microwaves are radiated continuously to a low-pressure range where reduced-pressure is large, a discharge phenomenon owing to Paschen's law occurs in a chamber, and microwaves are not absorbed into foodstuffs to be thawed, which has been still another problem with the prior art.
The present invention has been made in consideration of the problems in the prior art, accordingly, an object of the present invention is to provide a vacuum microwave thawing method and a vacuum microwave thawing machine that enables to eliminate the need to set the pressure restoration upper limit to a high-pressure range where reduced-pressure is smaller than required, and to prevent the edge effects the phenomenon of microwave concentration, and the discharge phenomenon owing to microwave irradiation.